I know your question is sincere but I suspect a little snark behind it. That's ok, Im pretty snarky myself so I will 'splain to you:
The 40% who aren't married occupy the lowest rungs of socio- economic society. If one wishes to join those ranks with babies in tow, go for it. I think that's stupid and irresponsible, I don't value poverty, but hey, those are MY values.
And that's what we are talking about here, values.
While a woman CAN reproduce without a man's commitment to his children as expressed by a legalized marriage ceremony, I think this cheats children out of important things. There are lots of details to this idea but
I'm giving you the executive summary.
Your idea that women should just want what you think they should have is silly.
Turning this on you, why don't you live with your girlfriend in her big house with all of her stuff,and get used to pooling your money with her so that you both can buy more crap and rack up the credit card debt? you don't because: that's not what you value.
Many women value a commitment as represented by a legally recognized public joining. On another thread we are talking about the legally supported financial institutions of society, one of them spousal Social Scurity benefits, and that's one among many reasons to get hitched legally.
I recognize that for some people, commitment without marriage is what they value. That's fine. I just think that in the vast majority of cases where they bring children into the world, kids are best served by two committed parents with legal protections.